Germ cell development is a complex, strictly ordered process of cell division and differentiation starting with the segregation of diploid primordial germ cells (PGCs) from the somatic lineage and finishing with the production of large numbers of mature haploid spermatozoa or oocytes. It is an essential determinant of fertility in all mammals, playing a fundamental role in the perpetuation of the species,, maintaining its genetic diversity and driving evolution. Recently, considerable progress has been made in identifying genes that play a role in the later meiotic stages of germ cell progression. However, very little is known about the pathways involved in the very earliest stages of PGC development. development. In fact, only four genes in the mouse have known, specific effects on PGCs (MGF, Kit, Ter, gcd). Mutations in each cause PGC deficiency and fertility problems. With the exception of gcd, these mutations are pleiotropic making an analysis of the specific germ cell component more difficult. The present proposal is designed to fill this gap in our knowledge, by identifying specific genes involved in the migration and proliferation of PGCs. The non-pleiotropic germ cell deficient (gcd) mouse mutant will be used as a model system. Analysis of this mutant clearly shows that disruption at a single locus, can drastically reduce the PGCs in the embryonic gonad, giving rise to male and female infertility. The male phenotype of a severe oligospermia quickly followed by azoospermia with only a few functional tubules, is very similar to the human Sertoli Cell Only syndrome (SCOS) seen in infertile human males. The specific aims of this project are designed to identify the gene underlying the gcd phenotyping using positional cloning and gene targeting. Its structure, spatio-temporal expression pattern and possible of studied by reintroducing the gene into gcd/gcd mice via transgenesis. That the cloning of gcd represents a unique opportunity to gain new insights into the fundamental biology of primordial germ lines and the intractable problem of infertility in humans. It will allow us to examine the possibility that errors in the GCD gene itself, or in a pathway which in controls, underlie a significant percentage of such cases in human.